<!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : --><chapterid="chap:hgext"><?dbhtmlfilename="adding-functionality-with-extensions.html"?><title>Adding functionality with extensions</title><paraid="x_4fe">While the core of Mercurial is quite complete from a
functionality standpoint, it's deliberately shorn of fancy
features. This approach of preserving simplicity keeps the
software easy to deal with for both maintainers and users.</para><paraid="x_4ff">However, Mercurial doesn't box you in with an inflexible
command set: you can add features to it as
<emphasis>extensions</emphasis> (sometimes known as
<emphasis>plugins</emphasis>). We've already discussed a few of
these extensions in earlier chapters.</para><itemizedlist><listitem><paraid="x_500"><xreflinkend="sec:tour-merge:fetch"/> covers the <literalrole="hg-ext">fetch</literal> extension;
this combines pulling new changes and merging them with local
changes into a single command, <commandrole="hg-ext-fetch">fetch</command>.</para></listitem><listitem><paraid="x_501">In <xreflinkend="chap:hook"/>, we covered
several extensions that are useful for hook-related
functionality: <literalrole="hg-ext">acl</literal> adds
access control lists; <literalrole="hg-ext">bugzilla</literal> adds integration with the
Bugzilla bug tracking system; and <literalrole="hg-ext">notify</literal> sends notification emails on
new changes.</para></listitem><listitem><paraid="x_502">The Mercurial Queues patch management extension is
so invaluable that it merits two chapters and an appendix all
to itself. <xreflinkend="chap:mq"/> covers the
basics; <xreflinkend="chap:mq-collab"/> discusses advanced topics;
and <xreflinkend="chap:mqref"/> goes into detail on
each
command.</para></listitem></itemizedlist><paraid="x_503">In this chapter, we'll cover some of the other extensions that
are available for Mercurial, and briefly touch on some of the
machinery you'll need to know about if you want to write an
extension of your own.</para><itemizedlist><listitem><paraid="x_504">In <xreflinkend="sec:hgext:inotify"/>,
we'll discuss the possibility of <emphasis>huge</emphasis> performance improvements using the <literalrole="hg-ext">inotify</literal> extension.</para></listitem></itemizedlist><sect1id="sec:hgext:inotify"><title>Improve performance with the <literalrole="hg-ext">inotify</literal> extension</title><paraid="x_505">Are you interested in having some of the most common
Mercurial operations run as much as a hundred times faster?
Read on!</para><paraid="x_506">Mercurial has great performance under normal circumstances.
For example, when you run the <commandrole="hg-cmd">hg
status</command> command, Mercurial has to scan almost every
directory and file in your repository so that it can display
file status. Many other Mercurial commands need to do the same
work behind the scenes; for example, the <commandrole="hg-cmd">hg diff</command> command uses the status
machinery to avoid doing an expensive comparison operation on
files that obviously haven't changed.</para><paraid="x_507">Because obtaining file status is crucial to good
performance, the authors of Mercurial have optimised this code
to within an inch of its life. However, there's no avoiding the
fact that when you run <commandrole="hg-cmd">hg
status</command>, Mercurial is going to have to perform at
least one expensive system call for each managed file to
determine whether it's changed since the last time Mercurial
checked. For a sufficiently large repository, this can take a
long time.</para><paraid="x_508">To put a number on the magnitude of this effect, I created a
repository containing 150,000 managed files. I timed <commandrole="hg-cmd">hg status</command> as taking ten seconds to
run, even when <emphasis>none</emphasis> of those files had been
modified.</para><paraid="x_509">Many modern operating systems contain a file notification
facility. If a program signs up to an appropriate service, the
operating system will notify it every time a file of interest is
created, modified, or deleted. On Linux systems, the kernel
component that does this is called
<literal>inotify</literal>.</para><paraid="x_50a">Mercurial's <literalrole="hg-ext">inotify</literal> extension talks to the kernel's <literal>inotify</literal> component to optimise <commandrole="hg-cmd">hg status</command> commands. The extension has two components. A daemon sits in
the background and receives notifications from the
<literal>inotify</literal> subsystem. It also listens for
connections from a regular Mercurial command. The extension
modifies Mercurial's behavior so that instead of scanning the
filesystem, it queries the daemon. Since the daemon has perfect
information about the state of the repository, it can respond
with a result instantaneously, avoiding the need to scan every
directory and file in the repository.</para><paraid="x_50b">Recall the ten seconds that I measured plain Mercurial as
taking to run <commandrole="hg-cmd">hg status</command> on a
150,000 file repository. With the <literalrole="hg-ext">inotify</literal> extension enabled, the time
dropped to 0.1 seconds, a factor of <emphasis>one
hundred</emphasis> faster.</para><paraid="x_50c">Before we continue, please pay attention to some
caveats.</para><itemizedlist><listitem><paraid="x_50d">The <literalrole="hg-ext">inotify</literal> extension is Linux-specific. Because it interfaces directly
to the Linux kernel's <literal>inotify</literal> subsystem,
it does not work on other operating systems.</para></listitem><listitem><paraid="x_50e">It should work on any Linux distribution that
was released after early 2005. Older distributions are
likely to have a kernel that lacks
<literal>inotify</literal>, or a version of
<literal>glibc</literal> that does not have the necessary
interfacing support.</para></listitem><listitem><paraid="x_50f">Not all filesystems are suitable for use with
the <literalrole="hg-ext">inotify</literal> extension.
Network filesystems such as NFS are a non-starter, for
example, particularly if you're running Mercurial on several
systems, all mounting the same network filesystem. The
kernel's <literal>inotify</literal> system has no way of
knowing about changes made on another system. Most local
filesystems (e.g. ext3, XFS, ReiserFS) should work
fine.</para></listitem></itemizedlist><paraid="x_510">The <literalrole="hg-ext">inotify</literal> extension is
shipped with Mercurial since 1.0.
All you need to do to enable the <literalrole="hg-ext">inotify</literal> extension is add an entry to
your <filenamerole="special">~/.hgrc</filename>.</para><programlisting>[extensions] inotify =</programlisting><paraid="x_51c">When the <literalrole="hg-ext">inotify</literal> extension
is enabled, Mercurial will automatically and transparently start
the status daemon the first time you run a command that needs
status in a repository. It runs one status daemon per
repository.</para><paraid="x_51d">The status daemon is started silently, and runs in the
background. If you look at a list of running processes after
you've enabled the <literalrole="hg-ext">inotify</literal> extension and run a few commands in different repositories,
you'll thus see a few <literal>hg</literal> processes sitting
around, waiting for updates from the kernel and queries from
Mercurial.</para><paraid="x_51e">The first time you run a Mercurial command in a repository
when you have the <literalrole="hg-ext">inotify</literal> extension enabled, it will run with about the same performance
as a normal Mercurial command. This is because the status
daemon needs to perform a normal status scan so that it has a
baseline against which to apply later updates from the kernel.
However, <emphasis>every</emphasis> subsequent command that does
any kind of status check should be noticeably faster on
repositories of even fairly modest size. Better yet, the bigger
your repository is, the greater a performance advantage you'll
see. The <literalrole="hg-ext">inotify</literal> daemon makes
status operations almost instantaneous on repositories of all
sizes!</para><paraid="x_51f">If you like, you can manually start a status daemon using
the <commandrole="hg-ext-inotify">inserve</command> command.
This gives you slightly finer control over how the daemon ought
to run. This command will of course only be available when the
<literalrole="hg-ext">inotify</literal> extension is
enabled.</para><paraid="x_520">When you're using the <literalrole="hg-ext">inotify</literal> extension, you should notice
<emphasis>no difference at all</emphasis> in Mercurial's
behavior, with the sole exception of status-related commands
running a whole lot faster than they used to. You should
specifically expect that commands will not print different
output; neither should they give different results. If either of
these situations occurs, please report a bug.</para></sect1><sect1id="sec:hgext:extdiff"><title>Flexible diff support with the <literalrole="hg-ext">extdiff</literal> extension</title><paraid="x_521">Mercurial's built-in <commandrole="hg-cmd">hg
diff</command> command outputs plaintext unified diffs.</para>&interaction.extdiff.diff;<paraid="x_522">If you would like to use an external tool to display
modifications, you'll want to use the <literalrole="hg-ext">extdiff</literal> extension. This will let you
use, for example, a graphical diff tool.</para><paraid="x_523">The <literalrole="hg-ext">extdiff</literal> extension is
bundled with Mercurial, so it's easy to set up. In the <literalrole="rc-extensions">extensions</literal> section of your
<filenamerole="special">~/.hgrc</filename>, simply add a
one-line entry to enable the extension.</para><programlisting>[extensions]
extdiff =</programlisting><paraid="x_524">This introduces a command named <commandrole="hg-ext-extdiff">extdiff</command>, which by default uses
your system's <command>diff</command> command to generate a
unified diff in the same form as the built-in <commandrole="hg-cmd">hg diff</command> command.</para>&interaction.extdiff.extdiff;<paraid="x_525">The result won't be exactly the same as with the built-in
<commandrole="hg-cmd">hg diff</command> variations, because the
output of <command>diff</command> varies from one system to
another, even when passed the same options.</para><paraid="x_526">As the <quote><literal>making snapshot</literal></quote> lines of output above imply, the <commandrole="hg-ext-extdiff">extdiff</command> command works by
creating two snapshots of your source tree. The first snapshot
is of the source revision; the second, of the target revision or
working directory. The <commandrole="hg-ext-extdiff">extdiff</command> command generates
these snapshots in a temporary directory, passes the name of
each directory to an external diff viewer, then deletes the
temporary directory. For efficiency, it only snapshots the
directories and files that have changed between the two
revisions.</para><paraid="x_527">Snapshot directory names have the same base name as your
repository. If your repository path is <filenameclass="directory">/quux/bar/foo</filename>, then <filenameclass="directory">foo</filename> will be the name of each
snapshot directory. Each snapshot directory name has its
changeset ID appended, if appropriate. If a snapshot is of
revision <literal>a631aca1083f</literal>, the directory will be
named <filenameclass="directory">foo.a631aca1083f</filename>.
A snapshot of the working directory won't have a changeset ID
appended, so it would just be <filenameclass="directory">foo</filename> in this example. To see what
this looks like in practice, look again at the <commandrole="hg-ext-extdiff">extdiff</command> example above. Notice
that the diff has the snapshot directory names embedded in its
header.</para><paraid="x_528">The <commandrole="hg-ext-extdiff">extdiff</command> command
accepts two important options. The <optionrole="hg-ext-extdiff-cmd-extdiff-opt">hg -p</option> option
lets you choose a program to view differences with, instead of
<command>diff</command>. With the <optionrole="hg-ext-extdiff-cmd-extdiff-opt">hg -o</option> option,
you can change the options that <commandrole="hg-ext-extdiff">extdiff</command> passes to the program
(by default, these options are
<quote><literal>-Npru</literal></quote>, which only make sense
if you're running <command>diff</command>). In other respects,
the <commandrole="hg-ext-extdiff">extdiff</command> command
acts similarly to the built-in <commandrole="hg-cmd">hg
diff</command> command: you use the same option names, syntax,
and arguments to specify the revisions you want, the files you
want, and so on.</para><paraid="x_529">As an example, here's how to run the normal system
<command>diff</command> command, getting it to generate context
diffs (using the <optionrole="cmd-opt-diff">-c</option> option)
instead of unified diffs, and five lines of context instead of
the default three (passing <literal>5</literal> as the argument
to the <optionrole="cmd-opt-diff">-C</option> option).</para>&interaction.extdiff.extdiff-ctx;<paraid="x_52a">Launching a visual diff tool is just as easy. Here's how to
launch the <command>kdiff3</command> viewer.</para><programlisting>hg extdiff -p kdiff3 -o</programlisting><paraid="x_52b">If your diff viewing command can't deal with directories,
you can easily work around this with a little scripting. For an
example of such scripting in action with the <literalrole="hg-ext">mq</literal> extension and the
<command>interdiff</command> command, see <xreflinkend="mq-collab:tips:interdiff"/>.</para><sect2><title>Defining command aliases</title><paraid="x_52c">It can be cumbersome to remember the options to both the
<commandrole="hg-ext-extdiff">extdiff</command> command and
the diff viewer you want to use, so the <literalrole="hg-ext">extdiff</literal> extension lets you define
<emphasis>new</emphasis> commands that will invoke your diff
viewer with exactly the right options.</para><paraid="x_52d">All you need to do is edit your <filenamerole="special">~/.hgrc</filename>, and add a section named
<literalrole="rc-extdiff">extdiff</literal>. Inside this
section, you can define multiple commands. Here's how to add
a <literal>kdiff3</literal> command. Once you've defined
this, you can type <quote><literal>hg kdiff3</literal></quote> and the <literalrole="hg-ext">extdiff</literal> extension
will run <command>kdiff3</command> for you.</para><programlisting>[extdiff]
cmd.kdiff3 =</programlisting><paraid="x_52e">If you leave the right hand side of the definition empty,
as above, the <literalrole="hg-ext">extdiff</literal> extension uses the name of the command you defined as the name
of the external program to run. But these names don't have to
be the same. Here, we define a command named
<quote><literal>hg wibble</literal></quote>, which runs
<command>kdiff3</command>.</para><programlisting>[extdiff]
cmd.wibble = kdiff3</programlisting><paraid="x_52f">You can also specify the default options that you want to
invoke your diff viewing program with. The prefix to use is
<quote><literal>opts.</literal></quote>, followed by the name
of the command to which the options apply. This example
defines a <quote><literal>hg vimdiff</literal></quote> command
that runs the <command>vim</command> editor's
<literal>DirDiff</literal> extension.</para><programlisting>[extdiff]
cmd.vimdiff = vim
opts.vimdiff = -f '+next' '+execute "DirDiff" argv(0) argv(1)'</programlisting></sect2></sect1><sect1id="sec:hgext:transplant"><title>Cherrypicking changes with the <literalrole="hg-ext">transplant</literal> extension</title><paraid="x_530">Need to have a long chat with Brendan about this.</para></sect1><sect1id="sec:hgext:patchbomb"><title>Send changes via email with the <literalrole="hg-ext">patchbomb</literal> extension</title><paraid="x_531">Many projects have a culture of <quote>change
review</quote>, in which people send their modifications to a
mailing list for others to read and comment on before they
commit the final version to a shared repository. Some projects
have people who act as gatekeepers; they apply changes from
other people to a repository to which those others don't have
access.</para><paraid="x_532">Mercurial makes it easy to send changes over email for
review or application, via its <literalrole="hg-ext">patchbomb</literal> extension. The extension is
so named because changes are formatted as patches, and it's usual
to send one changeset per email message. Sending a long series
of changes by email is thus much like <quote>bombing</quote> the
recipient's inbox, hence <quote>patchbomb</quote>.</para><paraid="x_533">As usual, the basic configuration of the <literalrole="hg-ext">patchbomb</literal> extension takes just one or
two lines in your <filenamerole="special"> /.hgrc</filename>.</para><programlisting>[extensions]
patchbomb =</programlisting><paraid="x_534">Once you've enabled the extension, you will have a new
command available, named <commandrole="hg-ext-patchbomb">email</command>.</para><paraid="x_535">The safest and best way to invoke the <commandrole="hg-ext-patchbomb">email</command> command is to
<emphasis>always</emphasis> run it first with the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -n</option> option.
This will show you what the command <emphasis>would</emphasis> send, without actually sending anything. Once you've had a
quick glance over the changes and verified that you are sending
the right ones, you can rerun the same command, with the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -n</option> option
removed.</para><paraid="x_536">The <commandrole="hg-ext-patchbomb">email</command> command
accepts the same kind of revision syntax as every other
Mercurial command. For example, this command will send every
revision between 7 and <literal>tip</literal>, inclusive.</para><programlisting>hg email -n 7:tip</programlisting><paraid="x_537">You can also specify a <emphasis>repository</emphasis> to
compare with. If you provide a repository but no revisions, the
<commandrole="hg-ext-patchbomb">email</command> command will
send all revisions in the local repository that are not present
in the remote repository. If you additionally specify revisions
or a branch name (the latter using the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -b</option> option),
this will constrain the revisions sent.</para><paraid="x_538">It's perfectly safe to run the <commandrole="hg-ext-patchbomb">email</command> command without the
names of the people you want to send to: if you do this, it will
just prompt you for those values interactively. (If you're
using a Linux or Unix-like system, you should have enhanced
<literal>readline</literal>-style editing capabilities when
entering those headers, too, which is useful.)</para><paraid="x_539">When you are sending just one revision, the <commandrole="hg-ext-patchbomb">email</command> command will by
default use the first line of the changeset description as the
subject of the single email message it sends.</para><paraid="x_53a">If you send multiple revisions, the <commandrole="hg-ext-patchbomb">email</command> command will usually
send one message per changeset. It will preface the series with
an introductory message, in which you should describe the
purpose of the series of changes you're sending.</para><sect2><title>Changing the behavior of patchbombs</title><paraid="x_53b">Not every project has exactly the same conventions for
sending changes in email; the <literalrole="hg-ext">patchbomb</literal> extension tries to
accommodate a number of variations through command line
options.</para><itemizedlist><listitem><paraid="x_53c">You can write a subject for the introductory
message on the command line using the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -s</option> option. This takes one argument, the text of the subject
to use.</para></listitem><listitem><paraid="x_53d">To change the email address from which the
messages originate, use the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -f</option> option. This takes one argument, the email address to
use.</para></listitem><listitem><paraid="x_53e">The default behavior is to send unified diffs
(see <xreflinkend="sec:mq:patch"/> for a
description of the
format), one per message. You can send a binary bundle
instead with the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -b</option> option.</para></listitem><listitem><paraid="x_53f">Unified diffs are normally prefaced with a
metadata header. You can omit this, and send unadorned
diffs, with the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg
--plain</option> option.</para></listitem><listitem><paraid="x_540">Diffs are normally sent <quote>inline</quote>,
in the same body part as the description of a patch. This
makes it easiest for the largest number of readers to
quote and respond to parts of a diff, as some mail clients
will only quote the first MIME body part in a message. If
you'd prefer to send the description and the diff in
separate body parts, use the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -a</option> option.</para></listitem><listitem><paraid="x_541">Instead of sending mail messages, you can
write them to an <literal>mbox</literal>-format mail
folder using the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -m</option> option. That option takes one argument, the name of the
file to write to.</para></listitem><listitem><paraid="x_542">If you would like to add a
<command>diffstat</command>-format summary to each patch,
and one to the introductory message, use the <optionrole="hg-ext-patchbomb-cmd-email-opt">hg -d</option> option. The <command>diffstat</command> command displays
a table containing the name of each file patched, the
number of lines affected, and a histogram showing how much
each file is modified. This gives readers a qualitative
glance at how complex a patch is.</para></listitem></itemizedlist></sect2></sect1></chapter><!--local variables: sgml-parent-document: ("00book.xml" "book" "chapter")end:-->